1. Field of the Invention
The invention relates to the manufacture of thin strips (of a thickness generally between 0.1 and 1.5 mm) made of aluminum-manganese alloy (3000 series according to the Aluminum Association designations), clad on one or two sides with an aluminum-silicon alloy (4000 series according to the Aluminum Association designations). Said strips are intended to manufacture heat exchanger deep drawn components assembled by brazing, said exchangers being used in particular in engine cooling and automobile body air conditioning systems, and more particularly for air conditioning unit evaporator plates. Brazing techniques for aluminum alloys are described for example in the article by J. C. Kucza, A. Uhry and J. C. Goussain “Le brasage fort de 1′aluminium et ses alliages”, published in Soudage et Techniques Connexes, November-December. 1991, pp. 18-29. The strips according to the invention are used particularly in non-corrosive flux brazing techniques such as NOCOLOK® or CAB (controlled atmosphere brazing), but may also be used in other brazing techniques such as vacuum brazing.
2. Description of Related Art
The use of aluminum alloys in automobile vehicle heat exchangers has developed in recent years, particularly due to the weight gain provided with reference to that of copper alloys. The properties required for the aluminum alloy strips used for the manufacture of brazed exchangers are particularly a good brazing capacity, a high mechanical resistance after brazing, such that the thinnest possible strips can be used, sufficient formability for easy shaping of the components, particularly evaporator plates comprising deep drawn ribs, and finally a good corrosion resistance. Said resistance is generally characterized by the SWAAT (salt water acetic acid test) test according to the standard ASTM G85. Naturally, it is important for the elaboration cost of the strips to be compatible with automobile industry requirements.
The alloy frequently used as the core alloy is 3003, composed as follows (% by weight according to the standard EN 573-3):                Si<0.6, Fe<0.7, Cu 0.05-0.20, Mn 1.0-1.5, Zn<0.10, other elements<0.05 each, and<0.15 in total, the remainder aluminum. Numerous alloys have been proposed in recent years to improve any of the properties for use mentioned above, more particularly corrosion resistance, hence the term “long-life” sometimes used to refer to said alloys in the related art.        
The patent EP 0326337 (Alcan International) describes a clad strip for which the core alloy is composed as follows:                Si<0.15, Fe<0.4, Cu 0.1-0.6, Mn 0.7-1.5, Mg<0.8        
The low Si content, preferentially<0.05%, enables the formation of a dense layer of Mn precipitates, frequently referred to as a “brown band”, which acts as a barrier to the diffusion of silicon in the coating alloy and increases corrosion resistance. WO 94/22633 is a variant of the above which only differs by a higher Cu content (0.6-0.9%) In both cases, the “brown band” can only be obtained in the absence of homogenization before hot rolling or intermediate annealing.
The patent EP 0718072 (Hoogovens Aluminum Walzprodukte) describes a core alloy composed as follows: Si>0.15, Fe<0.8, Cu 0.2-2, Mn 0.7-1.5, Mg 0.1-0.6, where Cu+Mg<0.7 and with the possible addition of Ti, Cr, Zr or V. The examples show Si contents of 0.5%.
The International patent application WO 99/55925 by the same company relates to an alloy composed as follows:                Mn 0.7-1.5, Cu: 0.5-1.5, Mg<0.8, Si<0.15, Fe<0.4        
In the brazed and aged state, the alloy shows a yield strength R0.2>75 MPa and a perforation-free service life in the SWAAT test of at least 13 days.
The French patent application by the applicant No. 99-10536 relates to strips or tubes for brazed exchangers made of alloy composed as follows:                Si 0.15-0.30, Fe<0.25, Cu 0.2-1.1, Mn: 1.0-1.4, Mg<0.4, Zn<0.2, Fe<Si, and Cu+Mg>0.4        
For parts requiring significant shaping, the strips are used in the annealed temper (O temper according to the standard NF EN 515) and in other cases in the cold-rolled temper, for example the H14 or H24 tempers.
The corrosion resistance of the brazed exchangers, as measured by the SWAAT test, depends not only on the composition of the core alloy or the selected brazing alloy. The phenomenon which practically always seems to cause rapid corrosion of exchangers and particularly of evaporator plates is liquid film migration or LFM. This phenomenon is described, for example, in the article by H. S. Yang and R. A. Woods “Mechanisms of Liquid Film Migration (LFM) in Aluminum Brazing Sheet”, VTMS3 Conference Proceedings, SAE International, Indianapolis, 1997, pp. 639-658. This consists of a diffusion process of the silicon from the brazing alloy to the core during brazing, the brazing alloy being either that clad on the core alloy strip or deposited on said strip by any other means, or obtained from the coating of the part adjacent to the brazing. This induces the formation of precipitate-rich grain boundaries, which form paths particularly liable to intergranular corrosion, due to the significant difference in potential between the phases present and the aluminum matrix. The presence of dislocations favours this phenomenon. This is one of the reasons, in addition to the improved formability, why an annealed temper is used, which results in a fine-grained recrystallized structure. However, for exchanger plates comprising deformed parts, shaping generates variable strain hardening in the part, and, to obtain a recrystallized microstructure throughout, it would be necessary to anneal the part after shaping, which would increase production costs. This is particularly the case for evaporator plates.